Boats are complex machines and understanding boat propeller exhausts is essential to maintain and fix them when necessary. It's important to take care of these exhaust systems because many of them are underwater systems.
The exhaust system of boats is often overlooked and not thought to be very important. The majority of boats use wet exhaust systems. This means that the waste gasses are mixed with salt water before leaving the engine. Preferably, the exhaust gasses should leave at the stern so that they do not interfere with the propeller.
Thru Hub Exhaust Systems
The majority of outboard and stern engines use thru-hub exhausts. You will be able to tell if you have a thru hub exhaust if there are holes around the shaft. These types of exhaust do not interfere with the water flow to the propellers, meaning that acceleration and speed are not compromised.
These are the most common types of exhaust system and are used in most engines. Other types of exhaust systems may be used on specialist engines.
Non Thru-Hub Exhaust Systems
Low horse powered engines normally use non thru-hub exhaust systems. You will be able to notice that you have this type of exhaust system if there aren't any holes around the prop shaft.
These will affect the speed of boats, which is why they are only used on low horsepower engines.
Over-Hub Exhaust System
These types of exhaust system are ideal for boats which need to travel at high speeds. They do not use a large exhaust tube because they have a much smaller tube which fits over the shaft. They create less drag, which will mean that the boat will achieve a higher speed.